Accelerometers are electromechanical devices that can measure acceleration forces due to motion and/or vibration. Accelerometers find use in a wide variety of applications, including seismic sensing, vibration sensing, inertial sensing and tilt sensing. Capacitive accelerometers are typically manufactured from silicon and implemented as micro electromechanical systems (MEMS) structures. A typical MEMS capacitive sensing structure comprises a proof mass moveably mounted relative to a support. A set of moveable electrode fingers extending from the proof mass are interdigitated with one or more sets of fixed electrode fingers, with differential capacitance between the electrode fingers being measurable so as to detect deflection of the proof mass in a sensing direction. An accelerometer comprising the sensing structure includes appropriate electronics for the drive and pickoff signals.
WO 2004/076340 and WO 2005/083451 provide examples of capacitive accelerometers comprising a plurality of interdigitated fixed and moveable electrode fingers extending substantially perpendicular to the sensing direction of the MEMS device. The electrode fingers are formed from a single silicon substrate, for example using deep reactive ion etching (DRIE). The silicon substrate is typically anodically bonded to a glass support with pre-cavitation of the glass where the elements move. After bonding and DRIE, a cap glass wafer is added to give a hermetic assembly with a gaseous medium trapped inside. The atmospheric pressure gas (typically argon) provides critical squeeze film damping for the proof mass when it moves. Down hole vias are then added to make electrical connection from the top surface of the glass support to the active silicon elements. The glass support offers high electrical insulation, but there is a mismatch in the coefficient of thermal expansion which depends on the glass type. For example, SD2 glass (alumino-silicate) gives a better thermal match than Pyrex (boro-silicate). Glass is used in preference to silicon (such as bonded silicon on oxide technology) for the support, as it reduces the stray capacitance to ground.
A prior art example of an accelerometer is described in WO 2012/076837 and seen in FIG. 1. In this sensing structure the proof mass is split into first and second mass elements arranged on opposite sides of a pair of fixed capacitor electrodes. The mass elements may be rigidly interconnected by a brace bar to form a unitary moveable proof mass. The mass elements are mounted to the underlying support by a set of four flexible legs that connect to separate top and bottom anchor points. The anchor points and the two sets of fixed electrode fingers are anodically bonded to the underlying glass support. A problem with this design is that differential expansion between the glass support and the silicon substrate arises when there is a temperature change. In the case of uniform thermal expansion, the two fixed electrodes move symmetrically (e.g. outwards) with respect to the two anchor points, which causes a scale factor shift as the electrode finger gaps are changed. In the case of a thermal gradient across the device, the two fixed electrodes move asymmetrically with respect to each other, resulting in a bias shift. Operating in open loop, the sensitivity of such a device is typically 30 nm/g (for a 30 g range), so 30 pm of relative movement gives rise to 1 mg of bias shift. Stressing of the support glass may result from thermal expansions/gradients, or be induced by the diebond used in the device package. The diebond is an elastomeric material with a low Young's modulus which normally has a high coefficient of thermal expansion and may also suffer from ageing effects.
The present disclosure seeks to reduce or overcome the disadvantages outlined above.